In beverage filling systems, a layer of foam is often formed during the filling in particular of beverages particularly containing carbon dioxide into beverage containers, such as bottles. Depending on the type of beverage, more or less foam can be formed. In particular during the filling of beer, a lot of foam is often formed. Normally, it takes some time until the foam has settled, i.e. until the liquid quantity bound in the foam is released. To check the liquid quantity in the beverage container, the filling level is measured. However, due to the forming foam, it is difficult, if not even impossible, to accurately determine the filling level directly after filling, because for doing so, one actually has to wait until the foam has settled. A determination of the liquid bound in the foam only on the basis of the dimensions of the foam often involves errors as the density of the forming foam varies, so that the bound liquid quantity does not only depend on the type of beverage but also varies from container to container.
If oxygen-sensitive beverages, such as beer, are filled, it is moreover common to inject a fine, sterile water jet into the filling orifice just in front of the closer (so-called HDE method). The foam generated in the process then displaces the oxygen still present above the filling level. To verify or check the correct function, it would also be desirable to know something about the foam density formed in the process.
With common methods, such as high frequency (HF), IR deflection, IR absorption, the liquid quantity in the foam cannot be determined. These methods have the disadvantage that they cannot detect different foam structures.
Though methods for measuring foam are known, one can only determine decomposition rates of the foams with these methods. In one of these methods, a light beam is emitted into the foam, where the light emerging from the foam on the opposite side is detected by a measuring device. The light beam and the measuring device rotate around the foam layer during the process. However, no exact determination of the foam density is possible with this method.